Goals: NASA's Viking mission to Mars was composed of two pairs of spacecraft -- Viking 1 and Viking 2 -- each consisting of an orbiter and a lander. The spacecraft were designed to take high-resolution images of the Martian surface, characterize the structure and composition of the atmosphere and surface, and search for evidence of life.

Accomplishments: Viking 1 was the first successful mission to land on Mars (the Soviet Mars 3 lander survived for a few seconds after landing in 1971, but sent back no science data). The Viking 1 lander operated on Chryse Planitia until November 1982. The four Viking spacecraft provided numerous new insights into the nature and history of Mars, producing a vivid overall picture of a cold weathered surface with reddish volcanic soil under a thin, dry carbon dioxide atmosphere, clear evidence for the existence of ancient river beds and vast floods, and no detectable seismic activity.

Although no traces of life were found, Viking found all elements essential to life on Earth -- carbon, nitrogen, hydrogen, oxygen and phosphorus -- were present on Mars.

The Viking landers took 4,500 unprecedented images of the surrounding surface and radioed more than 3 million weather-related measurements back to Earth, while their two companion orbiters took 52,000 images of sections of the Martian globe.

Viking 1 sent back the first image from the surface of Mars (right)

Viking 1 was supposed to be Viking 2, but the launch order was switched so engineers could fix a battery problem on the first spacecraft.